Hybrid organic–inorganic perovskite (HOIP) stability is affected by defects and moisture permeation. One strategy to overcome the latter is to use organic passivators, the choice of which is generally guided by chemical intuition learnt from steric principles. Through systematic study of the passivation of the CH3NH3PbI3 surface with different ammonium molecules, we introduce the concept of the Supramolecular Virtual Crystal (SVC) and propose a novel method to design the molecular passivation of the HOIP surface. We show that the interplay between steric effects and intermolecular interaction of the SVC ultimately determines the ideal passivating agent, which can be generalized to any hybrid perovskite. Two rules for optimal passivating-molecule design are proposed: (i) an aliphatic chain with more than one methylene group should be present; (ii) the optimized lattice parameter of the SVC should be similar to that of the chosen HOIP. Following these guidelines, we found that tert-butyl-terminated molecules are optimal passivating agents for CH3NH3PbI3.